In recent years there has been growing interest in the use of conjugated, semiconducting polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies up to 8%.
The conjugated polymer serves as the main absorber of the solar energy, therefore a low band gap is a basic requirement of the ideal polymer design to absorb the maximum of the solar spectrum. A commonly used strategy to provide conjugated polymers with narrow band gap is to utilize alternating copolymers consisting of both electron rich donor units and electron deficient acceptor units within the polymer backbone.
However, the conjugated polymers that have been suggested in prior art for use ion OPV devices do still suffer from certain drawbacks. For example many polymers suffer from limited solubility in commonly used organic solvents, which can inhibit their suitability for device manufacturing methods based on solution processing, or show only limited power conversion efficiency in OPV bulk-hetero-junction devices, or have only limited charge carrier mobility, or are difficult to synthesize and require synthesis methods which are unsuitable for mass production.
Therefore, there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing conjugated polymers containing benzo[1,2-b:4,5-b′]dithiophene (hereinafter shortly referred to as “BDT”) repeating units that are substituted in 4- and 8-position with an —X—CX—R group, hereinafter also referred to “reverse ester”, wherein X is O or S and R is carbyl or hydrocarbyl. It was found that polymers comprising such units are attractive candidates for photovoltaic applications, specifically in bulk heterojunction (BHJ) photovoltaic devices. By the incorporation of the electron-donating BDT unit and an electron-accepting unit into a copolymer i.e. a “donor-acceptor” polymer, a reduction of the bandgap can be achieved, which enables improved light harvesting properties in bulk heterojunction (BHJ) photovoltaic devices. Also, by modifying the BDT core unit with the addition of a reverse ester functionality at the 4- and 8-positions, respectively, the solubility and electronic properties of the copolymer can be further optimised.
U.S. Pat. No. 7,524,922 B2 discloses polymers comprising a BDT unit that is substituted in 4- and 8-position and/or in 3- and 7-position by optionally substituted groups like alkyl which may also contain an ester group.
WO 2010/135701 A1 discloses polymers comprising a BDT unit that is substituted in 4- and 8-position and/or in 3- and 7-position by a group like selected for example from a broad variety including H, CN, alkoxy, thioalkyl, ketone, ester, sulfonate, alkyl, alkenyl, alkynyl, haloalkyl, cycloalkyl, aryl, haloaryl, cycloheteroalkyl and heteroaryl.
However polymers with the specific structure as claimed in the present invention have not been not explicitly disclosed in prior art.